全文获取类型
收费全文 | 14938篇 |
免费 | 457篇 |
国内免费 | 322篇 |
专业分类
化学 | 9293篇 |
晶体学 | 104篇 |
力学 | 419篇 |
综合类 | 30篇 |
数学 | 2019篇 |
物理学 | 3852篇 |
出版年
2022年 | 115篇 |
2021年 | 190篇 |
2020年 | 184篇 |
2019年 | 180篇 |
2018年 | 176篇 |
2017年 | 141篇 |
2016年 | 339篇 |
2015年 | 307篇 |
2014年 | 367篇 |
2013年 | 801篇 |
2012年 | 638篇 |
2011年 | 738篇 |
2010年 | 529篇 |
2009年 | 511篇 |
2008年 | 677篇 |
2007年 | 720篇 |
2006年 | 587篇 |
2005年 | 604篇 |
2004年 | 592篇 |
2003年 | 586篇 |
2002年 | 629篇 |
2001年 | 445篇 |
2000年 | 407篇 |
1999年 | 314篇 |
1998年 | 225篇 |
1997年 | 207篇 |
1996年 | 229篇 |
1995年 | 241篇 |
1994年 | 234篇 |
1993年 | 180篇 |
1992年 | 171篇 |
1991年 | 180篇 |
1990年 | 192篇 |
1989年 | 158篇 |
1988年 | 144篇 |
1987年 | 138篇 |
1986年 | 130篇 |
1985年 | 222篇 |
1984年 | 238篇 |
1983年 | 165篇 |
1982年 | 181篇 |
1981年 | 153篇 |
1980年 | 156篇 |
1979年 | 146篇 |
1978年 | 124篇 |
1977年 | 166篇 |
1976年 | 144篇 |
1975年 | 122篇 |
1974年 | 113篇 |
1973年 | 115篇 |
排序方式: 共有10000条查询结果,搜索用时 358 毫秒
931.
Hue Thi Buu Bui Phuong Hong Nguyen Quan Minh Pham Hoa Phuong Tran De Quang Tran Hosun Jung Quang Vinh Hong Quoc Cuong Nguyen Quy Phu Nguyen Hieu Trong Le Su-Geun Yang 《Molecules (Basel, Switzerland)》2022,27(7)
Epigenetic alterations found in all human cancers are promising targets for anticancer therapy. In this sense, histone deacetylase inhibitors (HDACIs) are interesting anticancer agents that play an important role in the epigenetic regulation of cancer cells. Here, we report 15 novel hydroxamic acid-based histone deacetylase inhibitors with quinazolinone core structures. Five compounds exhibited antiproliferative activity with IC50 values of 3.4–37.8 µM. Compound 8 with a 2-mercaptoquinazolinone cap moiety displayed the highest antiproliferative efficacy against MCF-7 cells. For the HDAC6 target selectivity study, compound 8 displayed an IC50 value of 2.3 µM, which is 29.3 times higher than those of HDAC3, HDAC4, HDAC8, and HDAC11. Western blot assay proved that compound 8 strongly inhibited tubulin acetylation, a substrate of HDAC6. Compound 8 also displayed stronger inhibition activity against HDAC11 than the control drug Belinostat. The inhibitory mechanism of action of compound 8 on HDAC enzymes was then explored using molecular docking study. The data revealed a high binding affinity (−7.92 kcal/mol) of compound 8 toward HDAC6. In addition, dock pose analysis also proved that compound 8 might serve as a potent inhibitor of HDAC11. 相似文献
932.
Francesca Di Cristo Anna Valentino Ilenia De Luca Gianfranco Peluso Irene Bonadies Anna Calarco Anna Di Salle 《Molecules (Basel, Switzerland)》2022,27(7)
The management of periodontitis remains a vital clinical challenge due to the interplay between the microorganisms of the dental biofilm and the host inflammatory response leading to a degenerative process in the surrounding tissues. Quercetin (QUE), a natural flavonol found in many foods, including apples, onions and tea, has exhibited prolonged and strong antibiofilm and anti-inflammatory effects both in vitro and in vivo. However, its clinical application is limited by its poor stability and water solubility, as well as its low bioavailability. Thus, in the present study, electrospun polylactic acid (PLA) nanofibers loaded with different amounts (5–10% w/w) of QUE were produced to rapidly respond to the acidic microenvironment typical of periodontal pockets during periodontal disease. This strategy demonstrated that PLA-QUE membranes can act as a drug reservoir releasing high QUE concentrations in the presence of oral bacterial infection (pH < 5.5), and thus limiting Pseudomonas aeruginosa PAO1 and Streptococcus mutans biofilm maturation. In addition, released QUE exerts antioxidant and anti-inflammatory effects on P. gingivalis Lipopolysaccharide (LPS)-stimulated human gingival fibroblast (HGFs). The reported results confirmed that PLA-QUE membranes could inhibit subgingival biofilm maturation while reducing interleukin release, thereby limiting host inflammatory response. Overall, this study provided an effective pH-sensitive drug delivery system as a promising strategy for treating periodontitis. 相似文献
933.
It is shown that the introduction of glass fibres in the poly(butylene terephthalate) (PBT) matrix, increases its flammability. The mechanism of action of a typical fire retardant for PBT which is a mixture of a brominated organic compound and antimony trioxide, seems to involve both the condensed and the gas phase. 相似文献
934.
Water-soluble graft copolymers were synthesized by copolymerization of acrylamide with mono-methoxy-poly(ethylene oxide)-methacrylates (Me-PEO-MA). The Me-PEO-MA macromers were synthesized by a catalytic esterification of methacrylic acid with mono-methoxy-poly(ethylene glycol)s with different molecular weights. The graft copolymers obtained were characterized by 1H-NMR spectroscopy, gel permeation chromatography (GPC) and Ubbelohde viscosimetry. The rheological behaviour of aqueous polymer solutions, which are expected to show hydrophobic association at elevated temperatures, was studied with a cone and plate-rheometer. 相似文献
935.
Paolo Giusti Luigi Lazzeri Niccoletta Barbani Letizia Lelli Silvano De Petris Maria G. Cascone 《Macromolecular Symposia》1994,78(1):285-297
Blends of natural and synthetic polymers were studied for potential applications in the biomedical field. Collagen and hyaluronic acid were mixed in aqueous solution with poly(vinyl alcohol) and poly(acrylic acid). The properties of the blends were studied by differential scanning calorimetry and dynamic mechanical thermal analysis. Some methods were also investigated to enhance the miscibility of the polymers in these blends. 相似文献
936.
E.M.M. De Brabander-Van Den Berg A. Nijenhuis M. Mure J. Keulen R. Reintjens F. Vandenbooren B. Bosnian R. De Raat T. Frijns S. v.D. Wal M. Castelijns J. Put E. W. Meijer 《Macromolecular Symposia》1994,77(1):51-62
The synthesis and characterization of a new series of polypropylenimine dendrimers is reported. Using a repetition of the sequence of a Michael addition to a primary amine, followed by a heterogeneously catalyzed hydrogenation, ultra-pure polypropylenimine macromolecules with molecular weights up to 6912 are synthesized. The reaction sequence allows the preparation of these dendrimers at very large scales, whilst the availability of a simple purification in the sequence affords ultra-pure samples. The polypropylenimine dendrimers are fully characterized; apart from the first 0.5 generation they are all oils, possess a Tg in the range from −90 to −40 °C, are unexpectedly stable, and their intrinsic viscosity drops after generation 4. 相似文献
937.
Yvar Van Den Winkel Jeroen Van Der Laarse Frans J. J. De Kanter Thomas Van Der Does Friedrich Bickelhaupt Wilberth J. J. Smeets Anthony L. Spek 《Heteroatom Chemistry》1991,2(1):17-28
The synthesis of 2,2′-bis(bis(dimethylamino)-phosphino)-3,3′5,5′-tetra-tert-butylbiphennyl ( 5 ) is described. It was extensively studied by 1H, 13C, and 31P NMR spectroscopy. Furthermore, the X-ray analysis of 5 is reported. Crystals of 5 are tetragonal, space group P¯421c, a = b = 24.770 (3) Å, c = 12.658 (4) Å, Z = 8. The surprising reaction of 5 with proton acids leading to the formation of various phosphorus containing five- and six-membered ring compounds is discussed. On reaction of one of the six-membered ring compounds ( 9 ) with magnesium in THF, a λ3, λ5-diphosphaphenanthrene ( 19 ) was obtained. 相似文献
938.
Michele Lo Giudice Giuseppe Varone Cosimo Ieracitano Nadia Mammone Giovanbattista Gaspare Tripodi Edoardo Ferlazzo Sara Gasparini Umberto Aguglia Francesco Carlo Morabito 《Entropy (Basel, Switzerland)》2022,24(1)
The differential diagnosis of epileptic seizures (ES) and psychogenic non-epileptic seizures (PNES) may be difficult, due to the lack of distinctive clinical features. The interictal electroencephalographic (EEG) signal may also be normal in patients with ES. Innovative diagnostic tools that exploit non-linear EEG analysis and deep learning (DL) could provide important support to physicians for clinical diagnosis. In this work, 18 patients with new-onset ES (12 males, 6 females) and 18 patients with video-recorded PNES (2 males, 16 females) with normal interictal EEG at visual inspection were enrolled. None of them was taking psychotropic drugs. A convolutional neural network (CNN) scheme using DL classification was designed to classify the two categories of subjects (ES vs. PNES). The proposed architecture performs an EEG time-frequency transformation and a classification step with a CNN. The CNN was able to classify the EEG recordings of subjects with ES vs. subjects with PNES with 94.4% accuracy. CNN provided high performance in the assigned binary classification when compared to standard learning algorithms (multi-layer perceptron, support vector machine, linear discriminant analysis and quadratic discriminant analysis). In order to interpret how the CNN achieved this performance, information theoretical analysis was carried out. Specifically, the permutation entropy (PE) of the feature maps was evaluated and compared in the two classes. The achieved results, although preliminary, encourage the use of these innovative techniques to support neurologists in early diagnoses. 相似文献
939.
B. De Roover J. Devaux R. Legras 《Journal of polymer science. Part A, Polymer chemistry》1996,34(7):1195-1202
The homopolymerization of maleic anhydride was attempted at 190°C, during the melt-functionalization of polypropylene, either with or without organic peroxide using a Brabender plastograph. The free radical homopolymerization of pure maleic anhydride was also attempted either with or without organic peroxide, at 190°C, in vacuum-sealed glass vials. In all cases, free low molecular weight maleic anhydride oligomers were observed by low molecular weight size exclusion chromatography (SEC). This maleic anhydride homopolymerization tends to prove that the ceiling temperature of poly(maleic anhydride) probably lies above the previously published value of 160°C for these specific experimental conditions. © 1996 John Wiley & Sons, Inc. 相似文献
940.
F. J. Martínez Crespo J. A. Palop Y. Sainz S. Narro V. Senador M. Gonzlez A. Lpez De Cerin A. Monge E. Hamilton A. J. Barker 《Journal of heterocyclic chemistry》1996,33(6):1671-1677
Several quinoxaline 1,4-di-N-oxides have been shown to be efficient and selective cytotoxins for hypoxic cells. We present now a series of 4-cyano-2-oxo-1,2,4-oxadiazolo[2,3-a]quinoxaline 5-N-oxides 2a-2k . They were prepared starting from 3-amino-2-quinoxalinecarbonitrile 1,4-di-N-oxides 1a-1k and 2-chloroethyl isocyanate in dry dioxane at 100–110°. A reaction mechanism is proposed. The treatment of 1a with phenyl isocyanate afforded 2a . Reaction of 2c with silica gel yielded 1c . Compounds 2a-2g were heated in the presence of ethanol and 2-propanol giving the corresponding carbamates 3a-3g and 4a-4g . Compound 2d was already obtained by heating a mixture of 1d and ethyl chloroformiate. Compound 2b was prepared when the carbamate 3b was heated at 150°. Quinoxalines were tested as cytotoxic agents both in oxic and hypoxic cells. The most interesting compounds were 3g and 4g . 相似文献